Internal structural monitoring system

ABSTRACT

A improved method of monitoring a structure by mounting a sensor within a cavity of the structure to measure at least one of strain experienced by the structure and vibration experience by the structure. Mounting a wireless communication unit mounted within the structure and connecting the wireless communication unit to the sensor to receive data from the sensor and transmit the data to a receiver outside the structure. Mounting a power supply within the structure and connecting the power supply to the sensor and the wireless communication unit to supply necessary electrical power to the sensor and the communication unit.

This application claims the benefit of and incorporates by referenceU.S. Provisional Application No. 61/361,723 filed Jul. 6, 2010.

This invention was made with United States government support under U.S.Navy SBIR Award No. N68335-09-C-0176. The government has certain rightsin this invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to structural monitoringsystems, and more particularly to monitoring systems embedded within astructure that includes autonomous power supplies, sensors, and wirelesscommunication.

Structural monitoring is a known field used to examine the integrity ofstructures and predict when maintenance activities should be performed.Most conventional prior art in this field pertains to sensor systemswhich use sensors that are wired to a central data acquisition andprocessing unit. Structural monitoring systems have been appliedhistorically in situations where the cost to benefit ratio is low,because the sensor systems add significant cost, additional maintenancerequirements, and a level of reliability that may not coincide with thatof the structural system being monitored. For example, health monitoringsensors in power plants are often used because common failures costmillions of dollars in down time, while bridge monitoring sensors arenot typically employed because bridges rarely fail and sensorinstallations are costly. The cost-benefit ratio for each applicationdepends on both the cost savings that can be derived from early failureprediction and the lifecycle cost of the particular implementations ofthe structural monitoring system. The lifecycle cost for most monitoringsystems is in large part determined by the wired connections betweenremote sensor locations and a central data acquisition and processingunit. Wiring faults are common and are often difficult to diagnose,which increases the maintenance required to sustain the monitoringsystem. Wire installation costs are high because the wire often needs tobe protected.

In addition to wiring, the typical external mounting and means ofattachment of a monitoring device to a structure strongly influencestheir accuracy, performance stability and ultimately lifecycle cost ofsuch devices. Small degradation in adhesives or mechanical fastenerboundary conditions due to corrosion leads to changes in the operationof sensor and power supply, which can radically compromise the valuethat they offer. Changes to the sensor mount may require recalibrationof device, which erodes the maintenance reduction objectives of themonitoring system. Changes to the power supply mount can affect output,which compromises the performance of the whole monitoring system.External mounting of sensor systems presents further drawbacks in termsof vulnerability to external damage or tampering.

In the specific case of helicopter components such as rod ends,replacement is often initiated by the appearance of damage and blemishesin the external surface of the structural part. These blemishes mayresult from impacts with small debris during operation or frommaintenance activities in the proximity of the structural part.Considering the robustness of metal components relative to thevulnerability of an externally mounted device of a monitoring system,there is an issue of whether the devices of the monitoring system mustbe replaced more often than the structure that is being monitored.

A common problem with prior load sensors such as wire foil strain gaugesis induced noise from external electromagnetic sources. Foil straingauges can behave similar to an antenna, because they are typicallyexposed and mounted to a metal component. This is a particularlyrelevant issue in structural monitoring systems, if wirelesscommunication and wireless power transfer is used.

It is an object of the present invention to provide a structuralmonitoring system which can be embedded in a structure.

It is an object of the present invention to provide a structuralmonitoring system which does not require wired connections for power orcommunication.

It is an object of the present invention to provide a structuralmonitoring system with improved system robustness and reducedsusceptibility to tampering.

It is an object of the present invention to provide a structuralmonitoring system with reduced maintenance and improved useful life ofthe monitoring system.

SUMMARY OF THE INVENTION

A improved method of monitoring a structure by mounting a sensor withina cavity of the structure to measure at least one of strain experiencedby the structure and vibration experience by the structure. Mounting awireless communication unit mounted within the structure and connectingthe wireless communication unit to the sensor to receive data from thesensor and transmit the data to a receiver outside the structure.Mounting a power supply within the structure and connecting the powersupply to the sensor and the wireless communication unit to supplynecessary electrical power to the sensor and the communication unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view of an internal structural monitoringsystem embedded in a rod end according to the present invention.

FIG. 2 is a schematic representation of an internal structuralmonitoring system according to the present invention.

FIG. 3 is a schematic representation of an internal structuralmonitoring system embedded in a rod end according to the presentinvention.

FIG. 4 is a schematic representation of a load sensor embedded in astructure according to the present invention.

FIG. 5 is a schematic representation of a piezoelectric energy harvesterembedded in a structure according to the present invention.

FIG. 6 is a schematic representation of an inductive coupling forwireless power transfer according to the present invention.

FIG. 7 is a schematic representation of an approach for electromagneticradiation power transfer according to the present invention.

FIG. 8 is a schematic representation of an antenna mounted external to astructure according to the present invention.

FIG. 9 is a schematic representation of an antenna embedded in astructure according to the present invention.

DETAILED DESCRIPTION

The present invention is an internal structural monitoring system 8 fora structure as shown in FIG. 1. The present invention includes apparatusand methods associated with the internal structural monitoring system.The present invention includes structure modification and methods of howto modify a structure to receive the internal structural monitoringsystem. The present invention includes devices as part of the internalstructural monitoring system, how to install the devices of internalstructural monitoring system within a structure and how to use devicesof the internal structural monitoring system within a structure. Itfurther includes features to enable autonomous functionality without theuse of wired connectivity to a central processing unit.

The present invention provides an autonomous internal monitoring systemthat interrogates a structure or conditions to which the structure issubject. The present invention provides for measuring parameters whichcharacterize a structure's condition, use, or exposure to itsoperational environment without the use of external wires for power andcommunication. Such structural monitoring is used in support ofmaintenance activities. Structural monitoring is also used forevaluation of safety or control of a structure's use and performance.For example, monitoring of loading experienced by rod ends that are portof rods which interconnect components on helicopter main rotor assemblyprovides essential information regarding the accumulation of damage tothe rotor assembly. Knowing the accumulation of damage can helpdetermine the optimal time at which rotor assembly components should bereplaced to maintain a high level of safety, while minimizing vehiclemaintenance cost. In this specific example, structural monitoringprovides an essential basis for generating value in terms of reductionsin schedule and unscheduled maintenance, reduced work load, minimizationof down time, and increased safety.

The present invention provides for the use of low power sensors andwireless features for use in structural monitoring that can be used in abroader range of applications, while reducing the monitoring systemcost-benefit ratio. The low power consumption by a sensor, allows theinternal structural monitoring system to be powered over a long periodof time by devices such as small batteries or energy harvesters thatderive energy from their surroundings. Wireless features reduce the highliability of hardwiring devices of the internal structural monitoringsystem.

The present invention includes four primary subsystems of devices, whichinclude a sensor unit, master control circuit, wireless communicationunit, and power supply, as shown in FIG. 2. These subsystems functionintegrally for realizing the benefits of monitoring of structures whilethey are in use. The sensor unit includes one or more sensors tointerrogate the structure and environment surrounding the structure, inorder to determine the structure's condition, use, or exposure to itsenvironment. The sensors interface with the master control circuit forcontrol and data acquisition. Analysis and data compression of thesensor measurements is performed at the master control circuit, whichincludes a processor for such duties. The master control circuitcommunicates with the wireless communication unit to deliver thestructure's use data to a remote receiver location. The sensormeasurements are also stored locally in data storage at the mastercontrol circuit. The power supply uses energy harvester transducers,energy receivers, and energy storage devices to power the internalstructural monitoring system. The energy harvesters convert energyrelated to the structure's use or environment to electrical energy.During times when the structure is not in use or is stationary andstatically loaded, communication to or from the sensor may be required.The present invention provides for energy to be supplied to the internalstructural monitoring system using wireless energy transfer or an energystorage unit. Energy generated or received can be accumulated orpermanently stored in the energy storage unit, if available.

The term “structure” refers to the physical object that is the primarysubject that the internal structural monitoring system monitors andinterrogates. FIG. 2 shows a schematic of a monitoring system accordingto the present invention. FIG. 2 shows the four primary subsystems of asensor unit, master control circuit, wireless communication unit, andpower supply. The embedding of the devices of these subsystems withinthe structure and their corresponding particular implementations arepart of the present invention. It is desirable to have the foursubsystems collocated or tethered together in a close proximity of oneanother for various reasons. The sensor unit includes one or moresensors that measure motion, temperature, strain and load on thestructure. Motions can include vibration and acceleration. The mastercontrol circuit controls and communicates with each of the othersubsystems. The master control circuit includes an analog/digital (A/D)interface and digital input/output (I/O) for interfacing with thesensors, wireless radio and flash memory for data storage. The wirelesscommunication unit provides a wireless data link to a remote locationand includes a wireless radio and an antenna. Control and datatransmission to the radio is dictated by the master control circuit. Thepower supply functions to deliver and regulate electrical energy to allother subsystems. The power supply can include an energy harvester,wireless power receiver and energy storage unit. The implementation ofeach subsystem is described using an application for helicoptercomponents as an example. Specifically, the example shows the use of theinternal structural monitoring system in rod ends of rods that are usedas linkages.

FIG. 1 shows a schematic drawing of a rod end extending from the rodwith the internal structural monitoring system embedded within a cavity10 of a rod end shaft 12 of the rod end. The rod end is an idealplatform for the internal monitoring system. This is because the rodends are located at critical load links on main rotor dampers, pitchlinks or push rod assemblies of a helicopter. Together, the internalstructural monitoring system in the rod end supplement existinghelicopter Health and Usage Monitoring Systems (HUMS) by providing nearreal time load data on the rotor assembly. Since the rod end is a commonaircraft component, the internal structural monitoring system isapplicable to other locations on aircraft including landing gear. Rodends are also used as structural components on many other types ofvehicles and structures. The rod is usually a round tubular shape, butthe rod could be other shapes. The rod end includes an eye 14 used as aconnection point to other structural components. The rod end whenconnected to a rod is considered as the end of the rod. Extending fromthe eye is the rod end shaft 12. The rod end shaft 12 connects to therod and is typically threaded into an opening in the rod. The rod endshaft 12 of the present invention includes the cavity 10 to receive theinternal monitoring system. FIG. 3 shows a closer view of the internalstructural monitoring system in the rod end and how it can be applied toany type of structure that is designed to primarily receive axial loads.Example structures designed to primarily receive axial loads includerods, beams, tubes, struts, cables, and trusses. The structure toreceive the internal structural monitoring system should include sometype of cavity to receive and enclose the internal structural monitoringsystem. FIG. 3 shows a strain sensor, energy harvester, energy storage,master control circuit and RF circuit within the cavity 10 of the axialbody rod shaft 12 of the rod end. On the outside of the axial body is anenergy receiver that is linked to the energy storage of the power supplyand an antenna that is linked to the RF circuit of the wirelesscommunication unit.

FIG. 4 shows a detailed schematic view of a load sensor used as thestrain sensor shown in FIG. 3. FIG. 4 is one of many types of strainsensors that could be used. The load sensor is shown as a cylindricalshape within a cylindrical cavity of the rod end. The load sensor isplaced against the closed end 16 of the cylindrical cavity 10 which actsas a first system support. The load sensor is held in place with theclosed end 16 and a second system support 18, as shown in FIG. 3. Theload is applied along the axis of symmetry 20 of the cylindrical cavity10 from the close end of the cylindrical cavity 10 downward towards thesecond system support 18. Elongation or retraction of the structureabout the cylindrical cavity 10 will be registered as strain along axisof symmetry 20. The distance between the first system support 16 and thesecond system support 18 can be such that the load sensor is under astate of compression between the first system support 16 and the secondsystem support 18. The second system support 18 is mounted in thecylindrical cavity such that the second system support 18 moves with thestructure along the axis of symmetry 20. Because both the strain sensorand the structure have elastic compliance, a nominal state ofcompression on the load sensor enables it to register bidirectionalloading. Compression of the load sensor is preferably greater than themaximum tensile load that is intended to be measured. The alignment ofthe center axis of the load sensor with the center axis of thecylindrical structure, enables accurate load measurements even ifbending moments are introduced perpendicular to the structure's axis ofsymmetry. This is as opposed to sensor solutions mounted externally thatrequire multiple sensing locations and averaging algorithms to deducethe uniaxial loading. Spherical contacts at the top and/or bottom of theload sensor prevent transfer of bending from the structure to the loadsensor.

The sensor shown in FIG. 4 as an example uses a unique an axi-symmetricmagnetostrictive element and magnetic field sensor. The magnetic fieldsensor is shown as a Hall effect element for sensing changes in themagnetic field. FIG. 4 shows the Hall effect sensor mounted in thecenter of a cylindrical magnetostrictive element and permanent magnetslocated about the sides of the cylindrical magnetostrictive element.Changes in the load on the magnetostrictive element, cause changes themagnetic field in the proximity of the Hall effect sensor. The Halleffect sensor provides an electrical signal that is related to themagnetic field to which it is exposed. The sensor has a ferromagnetichousing mounted about the cylindrical magnetostrictive element. Theferromagnetic housing acts as a shield to protect against changes in themagnetic environment around the load sensor including the proximity offerromagnetic materials and other passive or active magnets which canstrongly influence the load sensor's sensitivity. For example, if theload sensor is mounted in a rod end on a helicopter and the load sensorpasses large steel fasteners as the blades rotate, the sensitivity couldvary, if susceptible to external magnetic environments because of lackof shielding. Because both the stiffness of the magnetostrictive elementand magnetostrictive responses of the magnetic field are nonlinear, itis advantageous to have their nonlinearity oppose each other. In thiscase, the magnetic field change due to displacement change of themagnetostrictive element tends to negate the high sensitivity of themagnetostricitve material for small strains. For large strains, thedeflection contribution is minimal and the magnetostrictive material isless sensitive to displacement change in this region, which results ingreater linearity over a wide range of strains.

FIG. 5 shows an example of an energy harvester power supply 22. Theenergy harvester 22 shown in FIG. 5 is a piezoelectric energy harvesterwith piezoelectric material. In this design, a piezoelectric transduceris directly attached or embedded in the host structure, where the hoststructure is the cylindrical cavity 10 of the rod end. The energyharvester 22 is positioned between the second system support 18 and athird system support 24 within the shaft 12 of the rod end. The secondsystem support 18 and a third system 24 support hold the energyharvester 22 in position, so that straining of the rod end causes thepiezoelectric material of piezoelectric element to strain with the rodend. Both the second system support 18 and a third system support 24 canbe positioned in the cylindrical cavity 10 by threading them into ahelical thread located on the inside of the cylindrical cavity 10. Boththe second system support 18 and a third system support 24 can be afastener with outside threads. There can be the case where the bottomsurface of the sensor acts as the second system support and the energyharvester 22 is positioned against the sensor, so that the sensor andthe energy harvester 22 are between only the first system support 16 andthird system support 24. Other methods than threads can be used to mountthe system supports 16, 18 within the host structure. Strain induced inthe piezoelectric element in turn generates electrical charge atelectrodes connected to the piezoelectric element. The charge isextracted by the master control circuit so it can be delivered to otherelectrical devices of the internal structural monitoring system. Sincethe electric field generated in the piezoelectric material is roughlyproportional to strain, the voltage output of the piezoelectric deviceis controlled by its thickness dimension. To maintain useable voltageswithout adding large step-down converters to the power managementcircuitry, the piezoelectric thickness must be small. Thin piezoelectriclayers are stacked to increase the strain energy in the piezoelectricmaterial which in turn increases the power generation capability of theenergy harvester 22. The piezoelectric stack element is coupledmechanically to the rod end by placing it under a preload using thesurrounding structure of the cylindrical cavity 10, the second systemsupport 18 and a third system support 24. In this way, the piezoelectricelement is in parallel with the primary stiffness of the rod end.Similarly to the load sensor, compression of the piezoelectric elementbetween second system support 18 and a third system support 24 ispreferably greater than the maximum tensile load that the rod end isdesigned to accommodate. The stack is preferably a cylindrical elementfitted into the cylindrical cavity 10 in the structure. The stack can bering shaped with a hollow center. It is preferably between 1 and 50 mmlong and between 5 and 50 mm in outside diameter.

Information recorded about the structure remains with the structure dueto the internal structural monitoring system being embedded within thestructure. In this case, removing the structure of the rod end from itsassembly will not disassociate it from the recorded data. This isimportant for air and land vehicle components that are periodicallyremoved, refurbished and may remain in storage for a period of time.Since retrieval of recorded information from the sensor depends on theoperation of the energy harvesting power supply, the power harvestermust be subject to its intended environment. When a structure with anembedded sensor and energy harvester is in storage or removed from itsassembly, the energy harvester is likely to be completely inert, makingit difficult to extract recorded information from the structure. This isan issue because one of the main purposes of embedded sensors is toevaluate if a structure can be returned to service. In this case, energymust be delivered to the sensor system while it is not in its intendedassembly. Since the sensor system is embedded and wireless, a wiredconnection for power would not be the most convenient solution. Energydelivery by wireless methods provides a simpler solution. FIG. 6 showsan inductive coupling method that is used to transfer energy from anexternal source using a resonant driver to a tuned resonant receiverthat is part of the internal structural monitoring system. Inductivecoupling as depicted in FIG. 6 involves generating a periodic magneticfield at the source and harnessing the electromotive force defined byFaraday's law to provide an electrical current in a receivingelectrically conductive element. By adding capacitance to the receivinginductive receiving element, electrical resonance is used to amplify theimpedance load and the power transfer. The power transfer is alsoenhanced for a given system by adding an additional set ofelectromagnetic resonators between the delivering and receiving coils.These resonators provide an impedance match condition that enablesefficient power transfer in a compact package. FIG. 7 shows a secondarypower transfer method that uses an alternating electric field to deliverpower from an external power generator to an energy receiver. This isgenerally achieved using electromagnetic radiation in the far fieldwhere the magnetic and electric fields are in phase. The electric fieldis generally initiated and received by antennas. Resonance is used toamplify the response of the antennas. The antennas include varioustopology including helical, Yagi, half wave dipole, quarter wave dipole,patch, collinear, conformal, fractal, aperture, etc.

Both the electromagnetic receiver for energy and the RF communicationcomponents, including antennas, require an unimpeded path for deliveringor receiving electromagnetic radiation. The structure is typicallyfabricated out of electrically conductive materials, which inhibit lineof site radiation. One solution is to use an antenna external to thestructure. This requires a small port through the structure to connectthe antenna to the electrical components that are embedded within thestructure. FIG. 8 shows an external antenna and a wiring conduit as thesmall port. FIG. 9 shows another solution by having a plug with a lowelectrically conductivity to enable the antenna components to beembedded and still deliver and receive electromagnetic radiation. Thecommunication antenna and electromagnetic energy receiver are preferablypotted or enclosed in packages having low electrical conductivity.

The present invention allows for monitoring structures in support ofmaintenance, control, or safety evaluations or activities related to thestructure. The present invention allows for sensing parameters relevantto monitoring the structure's use, condition, or exposure to itsenvironment. The present invention allows for storing the sensedparameters at or near the measurement location on the structure. Thepresent invention compresses and analyzes the measured parameters toextract useful information pertaining to the structure's use, condition,or exposure. The present invention transmits useful informationwirelessly from the monitoring location to a remote receiver. Thepresent invention allows for harvesting energy and supplying it to themonitoring system for operation when the structure's use or environmentsurrounding the structure provides an opportunity to generate energy.The present invention receives energy wirelessly from an external sourceor from an energy storage unit when the use or environment surroundingthe structure provides an insufficient opportunity to generate energy.

The present invention addresses essential features lacking in priorstate-of-the-art monitoring systems. In particular, it addresses therobustness and long term reliability issues described herein. Theapproach to addressing these issues involves integration of the sensorsystem directly with the host structure rather than simply attaching itto the host structure as is common in the prior art. The integrationinfluences the particular implementation of each of the subsystems. Itimproves the performance of both the sensor and the energy harvester. Italso reduces cost by eliminating expensive manufacturing stepsassociated with bonded piezoelectric fiber composites and foil straingauges. The system reduces maintenance costs associated with themonitoring system by providing a platform that does not require periodiccalibration or repair due to rigorous use. With these advancements,autonomous sensor nodes render a low cost and highly reliable structuralmonitoring solution, thereby expanding the opportunities for structuralmonitoring to a wide range of applications on vehicles, buildings, andother basic civilian infrastructure.

While different embodiments of the invention have been described indetail herein, it will be appreciated by those skilled in the art thatvarious modifications and alternatives to the embodiments could bedeveloped in light of the overall teachings of the disclosure.Accordingly, the particular arrangements are illustrative only and arenot limiting as to the scope of the invention that is to be given thefull breadth of any and all equivalents thereof.

We claim:
 1. An improved method of monitoring a rod end, comprising:using a rod end, the rod end having an eye used as a connection point toother structural components and a rod end shaft extending from the eye,the rod end shaft used for connection to a rod; mounting a sensor insidea cavity of the rod end shaft to measure at least one of strainexperienced by the rod end shaft and vibration experienced by the rodend shaft; mounting a wireless communication unit mounted inside thecavity of the rod end shaft and connecting the wireless communicationunit to the sensor to receive data from the sensor and transmit the datato a receiver outside the rod end shaft; and mounting a power supplyinside the cavity of the rod end shaft and connecting the power supplyto the sensor and the wireless communication unit to supply necessaryelectrical power to the sensor and the communication unit.
 2. Theimproved method of claim 1, further including mounting an energyharvester within the structure as part of said power supply.
 3. Theimproved method of claim 2, further including installing a third systemsupport in the structure and positioned in the structure such that theenergy harvester is between the second system support and the thirdsystem support to hold the energy harvester in position so that theenergy harvester can experience at least one of strain and vibration. 4.The improved method of claim 1, further including installing a firstsystem support and a second system support in the structure andpositioned in the structure such that the sensor is between the firstsystem support and the second system support to hold the sensor inposition so that the sensor senses at least one of strain and vibration.5. The improved method of claim 1, further including mounting the sensorso that the sensor is aligned axially along a length of the shaft of therod end so that the sensor experiences strain and vibrations that isexperienced at the eye of the rod end.
 6. An improved method ofmonitoring a structure, comprising mounting a sensor inside a cavity ofthe structure to measure at least one of strain experienced by thestructure and vibration experienced by the structure; mounting awireless communication unit mounted inside the structure and connectingthe wireless communication unit to the sensor to receive data from thesensor and transmit the data to a receiver outside the structure;mounting a power supply inside the structure and connecting the powersupply to the sensor and the wireless communication unit to supplynecessary electrical power to the sensor and the communication unit;further including installing a first system support and a second systemsupport in the structure and positioned in the structure such that thesensor is between the first system support and the second system supportto hold the sensor in position so that the sensor senses at least one ofstrain and vibration.
 7. The improved method of claim 6, furtherincluding mounting an energy harvester within the structure as part ofsaid power supply.
 8. The improved method of claim 7, further includinginstalling a third system support in the structure and positioned in thestructure such that the energy harvester is between the second systemsupport and the third system support to hold the energy harvester inposition so that the energy harvester can experience at least one ofstrain and vibration.
 9. An improved method of monitoring a structure,comprising creating a cavity inside a shaft of a rod end, wherein therod end includes an eye used as a connection point to other structuralcomponents and the shaft extends from the eye for connection to a rod;mounting a sensor inside the cavity of the structure to measure at leastone of strain experienced by the structure and vibration experienced bythe structure; mounting a wireless communication unit mounted inside thecavity of the structure and connecting the wireless communication unitto the sensor to receive data from the sensor and transmit the data to areceiver outside the structure; and mounting a power supply inside thecavity of the structure and connecting the power supply to the sensorand the wireless communication unit to supply necessary electrical powerto the sensor and the communication unit.
 10. The improved method ofclaim 9, further including mounting an energy harvester within thecavity of the structure as part of said power supply.
 11. The improvedmethod of claim 10, further including installing a first system supportand a second system support within the cavity of the structure andpositioned in the structure such that the sensor is between the firstsystem support and the second system support to hold the sensor inposition so that the sensor senses at least one of strain and vibration.12. The improved method of claim 11, further including installing athird system support within the cavity of the structure and positionedin the structure such that the energy harvester is between the secondsystem support and the third system support to hold the energy harvesterin position so that the energy harvester can experience at least one ofstrain and vibration.
 13. The improved method of claim 9, furtherincluding installing a first system support and a second system supportwithin the cavity of the structure and positioned in the structure suchthat the sensor is between the first system support and the secondsystem support to hold the sensor in position so that the sensor sensesat least one of strain and vibration.
 14. The improved method of claim9, further including mounting the sensor so that the sensor is alignedaxially along a length of the shaft of the rod end so that the sensorexperiences strain and vibrations that is experienced at the eye of therod end.